This document describes a proposal to study the aflatoxin contamination of different kinds of foods commonly consumed by Shanghai residents. At each season during a full year period, 20 samples of 17 kinds of food will be collected from three different market places and examined for aflatoxin contamination levels. The total number of food samples to be examined will be 4,080 (17 kinds X 3 markets X 4 seasons X 20 samples per season = 4,080). Aflatoxins will be measured by a variety of techniques utilizing TLC and HPLC, depending on the specific food item. Together with the dietary histories collected from Shanghai residents in a large ongoing cohort study, the results of the study will be used to estimate the amount of aflatoxin ingested by individual Shanghai residents, in order to determine the independent and interactive roles of aflatoxin and hepatitis B virus in the etiology of primary hepatocellular carcinoma.